Method of and apparatus for producing a variable alternating current



I Oct. 31, 1939. w PETERSEN 2,178,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENTFiled Aug. 12, 1938 8 Sheets-Sheet l Inventor: Waldemar Peters en,

His Attorneg. I

Oct. 31, 1939. w, PETERSEN 2,178,432 METHOD OF AND APPARATUS FORPRODUCING A VARIABLE ALTERNATING CURRENT Filed'Aug. 12, 1938 8Sheets-Sheet 2 His Attorney,

w. PETERSEN 8.432

Filed Aug. 12, 1938 8 Sheets-Sheet 3 [\I I A Inventor Waldemar Petersen,

b His Attorn e Oct. 31, 1939.

METHOD OF AND APPARATUS PRODUCING A VARIABLE ALTERNATING CURRENT 0d.31-, 1939. w. PETERSEN METHOD OF AND APPARATUS FOR PRODUCING A VARIABLEALTERNATING CURRENT 8 Sheets-Sheet 4 Filed Aug. 12, 1938 Inventor:Waldemar Petersen,

His ttor-neg.

1939- w. PETERSEN 7 3 METHOD OF AND APPARATUS FOR PRODUCING A VARIABLEALTERNATING CURRENT Filed Aug. 12, 1938 8 Sheets-Sheeti- Inventor:Waldemar" Petersen,

Oct. 31, 1939. w, PETER N 2,118,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENTFiled Aug. 12, 1938 8 Sheets-Sheet 6 Fig. IO.

Inventor: Waldemar- Petersen,

His Attorneg.

0a. 31, 1939. w. PETERSEN 2,178,432

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENTFiled Aug. 12, 1938 8 Sheets-Sheet 7 Inventor: Waldemar" Petersen,

Hi 5 At tor'ne Oct. 31, 1939. v w. PETERSEN ,178.62

METHOD OF AND APPARATUS FOR PRODUCING A VARIABLE ALTERNATING CURRENTFiled Aug. 12, 1938 8 Sheets-Sheet 8 4 4! Fig l4. 1 I 42 44 DDEIUDUUUDUDEI DDEIDUU C C b Inventor:

1 His ttorneg.

Patented Oct. 31, 1939 METHOD AND APPARATUS FOR PRODUC- ING A VARIABLEALTERNATING CUR- RENT Waldemar Petersen, Berlin, Germany, assignor toGeneral Electric Company, a corporation of New York Application August12, 1938, Serial No. 224,439 In Germany June 1, 1937 10. Claims.

My invention relates to the method of and the apparatus for producing avariable alternating current and more particularly to frequency changingsystems operated fromavariable ampli- 5 tude or frequency alternatingcurrent or from two alternating currents both of which are higher infrequency than the output current.

In certain instances such as motor drives for Pilger mills it isdesirable to obtain a variable speed or variable torque drive and forthis purpose there may be produced a variable frequency or variableamplitude alternating current which is supplied to the motor in order toobtain the desired speed torque characteristic. Particularly 1b where itis desired to transmit energy from a higher frequency alternatingcurrent circuit to a lower frequency alternating current circuit bymeans of frequency changers it will be found that frequency changers ofthe type utilizing electric :10 valve converting system will haveimproved operation characteristics if these valve arrangements areenergized from a source of variable frequency alternating current. Inaccordance with my invention such variable frequency alternatingcurrents are obtained by adding together the voltages obtained from twoalternating currents of different frequency. When these voltages areutilized to supply an electric valve frequency changing arrangement theoutput frequency will "so be an alternating current having a frequencyequal to half the difference between the two alternating current supplyfrequencies.

It is an object of my invention to provide a method of and an apparatusfor producing a variable frequency alternating current which will bereliable and simple in operation.

Another object of my invention is to provide an improved frequencychanger apparatus operating to transmit energy betwen two or more 0alternating current circuits.

Tne novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organiza- 45 tion and methodofoperation, together with further objects and advantages thereof, will bebetter understood by reference to the following description taken inconnection with the accompanying drawings in which Figs. 1 to 6illustrate graphically the method of producing variable frequencyalternating currents; Fig. 7 shows a circuit arrangement by means ofwhich my method may be carried out; Fig. 8 shows an electric valvefrequency changing apparatus operating 55 in accordance with myinvention; Fig. 9 shows a further modification of the arrangement shownin Fig. 8; Figs. 10 and 11 are further examples of frequency changingapparatus embodying the principles of my invention; Fig. 12 graphicallyillustrates the application of my invention to frequency changingapparatus utilizing zero or neutral point anodes; and Figs. 13 and 14show further frequency changing apparatus to which my invention has beenapplied.

Referring to Fig. 1 of the drawings I have shown therein a graphicalrepresentation of how the superimposition of two sine waves ofapproximately like frequency will produce a resultant wave whosefrequency is the arithmetical mean of the sum of the frequencies of thetwo alternating waves. This resultant wave indicated as Eab in Fig. 1has periodic variations in amplitude which are commonly called beats. Asis well known, two resultant beat frequencies" are obtained by suchsuperimposition of two sine waves having the same amplitudes andslightly different frequencies, namely, one having a frequency equal tothe difference between the original frequencies and the other having afrequency equal to the sum of the original frequencies. Since a beatcycle is defined as one complete rise and fall in amplitude, Fig. 1discloses one beat cycle of a frequency equal to the difference betweenthe two frequencies of the superimposed waves which is quite obviousfrom a study of Fig. 2a.. The locus of the amplitudes of the resultantcurve Eab describes an envelope Eu Figs. 1 and 4, which varies insinusoidal form if one considers alternate positive and negative loci ofthe amplitudes of successive beat cycles and the frequency of thissinusoidal envelope is equal to half the difference between thefrequencies of the two alternating current waves which produce theresultant wave Eab. Where the envelope is of sinusoidal form it is to beunderstood that this resultant envelope is obtained from thesuperimposition of two sinusoidal curves having equal amplitude.Mathematically the values of such curves or voltages are added inaccordancewith the following formula:

Eab=2A sin Trt(fa+fb) -cos 1rt(fafb) :ZA sin obtained-by superimposingEb on Ea, w is the circuit frequency in radians per second, 1 is thefrequency in cycles per second and A is the amplitude of the voltagesE9. and Eb. It is observed that the resulting wave Eab contains aproduct of two trigonometric functions. Considering the coefficient ofthe cosine term in the above derived equations, it is observed that thevalue of this term becomes zero at successive values of t differing byvalues of or one-half of the beat frequency and the amplitude variesbetween the value 2A and 0. Therefore the alternating wave Eu thusobtained contains two beat cycles for each cycle of Eu as is quiteobvious from Fig. 3 since a beat cycle extends from To to T1 while acomplete cycle of the alternating wave Eu extends from To to T2,. ItWill be seen that the frequency of the envelope Eu is considerably lessthan either of the frequencies of the alternating waves Ea and Eb. Thismethod of lowering the frequency has the disadvantage that a number ofgaps exist in the energy flow. This is apparent from Fig. 1 by observingthe envelope Eu of the resultant voltage Eab. Thus there are energy gapsduring the time T1, T3, T5, etc. relative to the upper limit of thecurve Eu. Where it is necessary to deliver a large energy output, suchwave shapes would therefore be undesirable. It therefore, would benecessary to supply additional energy during these periods and suchenergy supply would also operate at a frequency equal to one-half of thesum of the frequencies of the supply circuit and at the same time a beatwould result having a frequency equal to the difference of the twosupply frequencies.

In accordance with my invention it is possible to reduce the frequencyby combining two alternating current waves and yet at the same timemaintain a substantially continuous energy flow. This is accomplished bysuperimposing the alternating voltage or current waves of a plurality ofpolyphase angularly symmetrical systems having a like direction ofrotation and which Waves have approximately like amplitude and similarwave shape. The degree of continuity in the energy flow with referenceto the envelope Eu may be increased by increasing the number of phasesused. In the instance of three phases of sine wave form the values areadded in accordance with the following mathematical formulae:

where Ea, E's. and E"& are the instantaneous volt- E"b are instantaneousphase voltages of another three-phase alternating curent voltage of aslightly different frequency and a, a and a, b, b and b" indicate thephase displacement in radians of the individual phase voltages comparedwith a starting point which is assumed to be zero. By superimposing Eaand Eb, E's and Eb, and Ea and E"b the following equations are obtained:

cos o F 1 pa n Let Then - Figs. 2A to 20. Figs. 2A to 20 differ fromeach other in that the voltages Ea, E'a, E".. and the voltages Eb, E'b,E"b are displaced 120 with respect to each other. The addition of thesetwo voltages produces a resultant voltage Eab, E'ab, and E"ab, each ofwhich has a similar envelope Eu as is apparent from Figs. 2A to 20. InFig. 3 there is shown the manner in which the three phase voltages arecombined. From this figure it willbe apparent that at the time T0, T1,T2, etc., the voltages Ea and Eb have an instantaneous value which iszero with the result that the resultant voltage Eab is zero. At the sametime the remaining voltages E'a and Eb and E"a and E"b haveinstantaneous values which are of equal value in opposite phase so thatthe resultant values E'ab Or E"ab at the time T0, T1, T2, etc., arelikewise equal to zero. These conditions can also be verified byreference to the mathematical Equation (2), which give the standards forthe reduction of frequency in the cosine function. This function isidentical for all three phases Eab, E' s and E"ab when the dfferences ofthe phase angles a and b is zero or of equal value for all phases. Fromthis it follows that the displacement of the individual voltages whichare added together in multiphase must be the same in each system withreference to the actual frequency scale.

The mathematical Formula (2) also gives an indication as to the mannerin which the resultant envelopes Eu are displaced in phase relation withrespect to each other in multiphase systems. The factor 11 which occursin Equation (2) indicates the difference in phase in radians between theindividual voltages participating in the voltage formation as comparedwith the initial value. If measures are taken so that the values a and bdiffer from each other by certain predetermined angular amount, that is,in contradistinction to the previous requirements of equality, thenenvelopes will be obtained which have a different phase position. Thus,for example, if it were desired to obtain a system having envelopescorresponding to a three-phase system one would displace one of the twoproducing voltages by 180 and then the phase displacement of theenvelope would be 90. Fig. 4 illustrates graphically how this occurs. Ifthe voltages represented by the curves Eao' which comprise theindividual voltages Ea, E'a, E"a, are combined with the voltages of .thecurves Ebo which comprise the individual voltages Es, E's, E"b, anenvelope Eu corresponding to the fourth curve in this figure would beobtained. If now the voltages similar to the curves E are combined withvoltages shown by the curves Eco which are 180 out of phase with respectto the curves Ebo then the envelope E'u forming the last portion of Fig.4 would be obtained which envelope has a phase displacement of 90 withrespect to the envelope Eu. The same results would follow if instead ofdisplacing the voltages Ebo by 180 as shown by Eco, the voltage E80 weredisplaced by the same amount. It will be apparent to those skilled inthe art that this illustration is not limited to the three-phase examplejust described but can be applied equally well to other multiphasearrangements.

While for the purposes of illustration up to this point it has beenshown that sinusoidal voltages may be added by these methods, it is alsopossible to combine multiphase voltages having non-sinusoidal waveshapes. These non-sinusoidal wave shapes when added, however, willproduce similar shaped non-sinusoidal envelopes.

The only limitation to be observed is that the higher harmonics shouldnot exhibit any phase displacement relative to thefundamental wave withrespect to the passage through zero of the latter wave and that bothforces of energy should "have mutually equal mean symmetrical waveshapes. This is adequately illustrated by the graphical representationof Figs. 5 and 6 which show the possibilities of adding non-sinusoidal vwave shapes such as triangular waves and trapea a); b b wherein thefunction 1 (wt) fluctuates with the circuit frequency w between thevalues +1 and 1; A is the amplitude which is the same for I bothalternating current voltages. In comparison to the sinusoidal form thisformula is developed into a Fourier series the coefficients of which (afor a system with a frequency, of wt, b for a system with a frequency of10b) assume such values that Ea and Eb for different time instants aregiven values corresponding to those of the triangular or trapezoidalshape. Such a phase is represented by the following equation:

This simple equation .is' obtained by assuming that the higher harmonicsdo not exhibit any phase displacement relative to the fundamental wave.If a series is set up for each of the two producing functions oralternating current sources, then these will differ only in the circuitfrequencies 10a and wt and in the phase angles a and s but not in thevalues of the Fourier coefiicients an to an and be to bu. From this itis apparent that it is necessary to have two wave shapes which aresimilar with the exception of their phase displacement. If now two suchwaves are added, which waves are produced by the individual currents orvoltages E9. and Eb, then the following formula'is obtained:

+2.13 sin wm m) cos wd+ 4)+ If the first and last members oftheresulting voltages Eab to E'Hm are combined, then there will beobtained voltages which have cosine functions with the frequency wa andits multiples as a limit and which exhibits an associated phase angle ofd and its multiples. The combination of the individual limits then takeplace} in the same manner as in the case of the funda inental functionsEa and Eb. The intermediate quantities, the sine functions, indicate thecourse of the voltages between the envelopes. As has been stated, curveforms similar to the fundamental wave shapes are only obtained when bothfundamental components or. voltages have similar forms, since. otherwisethe Fourier coeflicients or to a and In to ba would differ from eachother and it would not be possible to add in the individual harmoniccomponents, or a zero value frequency change may be obtained which willhave a wave shape satisfactory for all purposes, without the use ofadditional or auxiliary energy supply forces or energy storage devices,by adding together two polyphase voltages of different frequencies ofany one of a large number of symmetrical wave shapes.

The resultant voltages obtained by the addition of a plurality ofvoltages of different frequencies to produce an envelope which variesperiodically in accordance with this invention may be utilized for powerpurposes. A number of examples showing the manner in which suchresultant voltages having an envelope of a relatively low frequency maybe used will now be explained. The arrangement shown in Fig. 7 shows asimple form of frequency changer operated from an alternating currentsource or line l which has a frequency equal to fa which supplies energyto a transformer 2 having a star connected secondary winding 3. Asuitable motor 4 is utilized to drive an alternating current generator 5which has a frequency fb which is supplied to the primary winding of atransformer 6, the secondary winding 1 of which is connected in serieswith the secondary winding 3 of the transformer 2 and the resultantvoltages are supplied to the alternating current cirwit 8, which has afrequency-which varies so as to have an envelope of a frequency fu- Thevoltages combined in the transformer secondaries 3 and 7 are similar tothe graphical representations of voltages disclosed in Figures 2A to 2Cand Fig. 3. If a three phase motor is now connected to the alternatingcurrent circuit 8 the torque thereof will be varied in accordance withthe frequency difference between the two voltages fa and ft. Thedifference between the frequencies, however, is not apparent in theoperation of the motor since the torque is proportional to the square ofthe voltage or to the square of the current. As will be apparent fromformula 1 the motor will operate at a frequency which is equal to thearithmetical mean of the sum of the frequencies of the two supplyvoltages. If the moment of inertia of the motor is not/too great or ifthe motor is not connected to an extremely heavy load, then a change inspeed will occur which i has a rhythm corresponding to the frequencydifference between the two supply voltages. Such motors may be used withadvantage to drive Pilger mills and any other drives which requiresimilar movement as for instance drives for riddles and certainfeedmovements in seam welding operation.

A further important field of use to which the idea of this invention maybe applied is found in those instances where it is advantageous tosupply 'single or multiphase low frequency systems from a multiphasesystem of higher frequency or where.

it is desired to transfer energy between two such systems. Sincerotating machines have certain disadvantages static electric valveconverting devices have been utilized. By means of my invention it'isnow possible in a simple manner to; satisfy all the requirements ofconverter operation and asynchronous connection between alternatingcurrent systems. Heretofore the wave shape produced by electric valveconverting frequency changing systems have had the disadvantage ofproducing wave shapes which were nonsinusoidal in form. Numerous methodshave been su gested, such as tap transformers, for improving thelwaveshape of such systems. The disadvantages of such additionalcomplicationsand apparatus may be obviated by the application of my to cycles.

invention to electric valve converting apparatus. In Fig. 3 reference ismade to the possibility of reducing the frequency by producing aresultant wave having an envelope Eu. From this it is apparent thatthere is only the problem of converting such resultant waves into a purealternating current wave. The resultant polyphase voltages willtherefore be supplied to an electric valve converting apparatus in whichone group of discharge devices will operate to produce a positive halfwave and another group will operate to produce a negative half wave.Such an arrangement is disclosed in Fig. 8 wherein the polyphasealternating current supply line 9 is used to supply energy to a motor H)which is coupled to an alternating current generator II. It will beassumed that the supply line has a frequency equal to It and the outputof the generator H has a frequency equal to fb. The generator I isconnected to a transformer having a primary winding l2 and a pluralityof groups of secondary windings I3 and H. The alternating current supplyline 9 is also connected to a transformer having a primary winding l5and a plurality of groups of secondary windings IS and. IT. Thesecondary winding l6 of the one transformer and the secondary winding |3of the other transformer are connected in series and similarly thesecondary winding ll of the one transformer and the secondary winding Mof the other transformer are likewise connected in series. These twosets of series connected secondary windings of the transformers operateas two polyphase networks supplying voltages to two groups of arcdischarge paths which may comprise a single cathode, multi-anodeelectric discharge device -|8. Any of the types of electric valves wellknown in the art may be utilized although it is preferable to use thosehaving an anode and a cathode contained within an envelope filled withan ionizable medium. The cathode of the electric discharge path I 8 isconnected through a suitable reactor I9 to the midpoint of the primarywinding of a transformer 20 the secondary winding of which is connectedto the alternating current load circuit 2|. One extremity of the primarywinding of the transformer 20 is connected to the neutral point of thetransformer secondary winding l6 and the other extremity of the primarywinding of the transformer 20 is connected to the neutral point of thetransformer secondary winding Thus when the voltages which resultfrom'the voltage component. introduced by the transformer primarywindings l2 and I5 are such as to produce positive resulting potentialsin the network comprising the secondary windings 3 and 6 the various arcdischarge paths associated therewith will becomeconductive to supply tothe transformer 20 energy serving to produce one-half wave of thealternating current appearing in the output circuit 2|. If, for example,it is shown that the alternating current circuit 9 has a commercialfrequency of 60 cycles and it is desired to supply 25 cycles to thealternating current output circuit 2|, the frequency of the generator II will be equal Obviously of course it is immaterial as towhether thealternating current supply line or the generator. H is of a lowerfrequency since it is only necessary t o supply to the two transformersecondary networks ico'mprising the poly- A phase networks of theelectric valve converting apparatus twp frequencies which differ by asum equal to twice the frequency to be transmitted to the alternatingcurrent load circuit 2|. -If "desired, the alternating current suppliedby the generator may be variable in frequency and hence nonconductive.

the alternating current in the output circuit 2| will be variable infrequency. For this purpose I have schematically illustrated motor ID asof the wound rotor induction motor type havinga variable resistor lllsconnected to the rotor terminals I whereby variation of the speed ofmotor l may be obtained and hence also variation of the output frequencyof generator H. The various arc discharge paths of the electric valvesmay be controlled by control grids. Any suitable control circuit wellknown in the art may be utilized in conjunction with the controlelectrodes or grids. For example, in Fig. 8 I have shown the well knowncontrol circuit described and illustrated in United States LettersPatent No. 1,408,118, grantedFebruary 28, 1922, to F. W. Meyer. Thecontrol electrodes or grids 66 for the discharge paths associated withthe secondary windings l4 and H are energized with a positive potentialfrom battery 61 through distributor 68 and resistances 69 and 10 every180 electrical degrees with reference to a voltage cycle of the outputfrequency I; that is,

Il -f1, 2

During the other half cycle control electrodes or grids H for thedischarge paths associated with secondary windings l3 and iii areenergized with positive potentials from battery 12 through distributor13 and resistances I4 and 15. A negative bias potential from batteries16 and TI is impressed upon control electrodes 66 and II, respectively,during the alternate half cycles for which the associated dischargepaths are to be maintained Distributors 68 and 13 may be driven by asynchronous motor 16 energized from a suitable source of supply 19having a frequency equal to half the difference between frequencies faand It. If the frequency changer illustrated in Fig. 8 is operating,synchronous motor 18 may be energized directly from the output circuit21. It will be understood by those skilled in the art that any othercontrol circuit well known in the art may be used where only a singlefrequency is utilized for controlling the arc discharge paths and thisfrequency corresponds to the alternating current output frequency.During one half cycle of the output frequency in control electrodes 66are energized with positive potential and the three associated dischargepaths become successively conductive twice during each half cycle asillus trated by Fig. 12. The current commutates from one discharge pathto the other whenever the voltage impressed on the succeeding anode tobecome conductive is equal to or higher than the voltage impressed uponthe anode of the discharge path which is conducting current. At the endof the half cycle the negative bias impressed upon control electrodes 66by virtue of bias battery 16 maintains the associated discharge pathsnonconductive and during the next half cycle the discharge pathsassociated with control electrodes H conduct current so as to produce aresultant output voltage of a frequency equal to half the differencebetween the two supply frequencies.

If desired the arrangement shown in Fig. 8 may be modified in accordancewith that shown in Fig. 9 wherein similar parts have been given similarreference characters. In this instance, however, the motor I0 is of thesynchronous type and the generator. i I has been replaced by a generator22 which is of the double winding type. A set of windings of thegenerator is in series with a different set of secondary windings suchas l6 and M, respectively, of the transformer the primary winding l5 ofwhich is connected to the alternating current circuit 9. As shown hereit would be preferable to connect the extremities of the outputtransformer 20 to the neutral points of the two groups of windings 23and 24 of the machine or generator 22 in order to simplify the number ofconnections to the generator.

Still another system to which my invention may be applied is shown inFig. 10. An alternating current supply circuit 25 supplies energy to atransformer having a primary winding 26 and a secondary winding 21. Thecircuit also supplies energy to a motor 28 which drives a generator 29the output of which is connected to a transformer having a primarywinding 30 and a secondary winding 3|. The alternating current supplycircuit will be assumed to have a frequency equal to fa and thegenerator 29 will be assumed to have a frequency equal to it. The twosecondary windings 21 and 3| of the transformers are connected inseries, the one being connected in star relation so that the neutralpoint thereof is connected to one side of the alternating current load32. remaining connections to the secondary windings 21 and 3| are eachconnected by a pair of oppositely connected valves such as 33, 34; 35, 36; 31, 38; to the other side of the alternating current load 32. For thepurposes of illustration each of the valves 33 to 38 have been shown ascomprising an anode, a cathode and a control grid contained within anenvelope filled with an ionizable medium, but it will be understood thatany other valves commonly known in the art may be utilized. Any suitablecontrol circuit which is well known in the art may be used to renderconductive these valves in proper sequence so as to transmit to thealternating current'load 32 alternating current energy having afrequency equal to one-half of the difference between the frequencies faand is.

Fig. 11 shows another embodiment similar to that shown in Fig. 8 andhence similar parts have been given similar reference characters. Inthis arrangement, however, the electric arc discharge paths have beenarranged in two groups l8a and I81). Both the neutral points of thetransformer secondary groups I6 and I! have been connected together tothe midpoint of the primary winding of the transformer 20. The outerextremities of the transformer primary winding have each been connectedto a different one of the cathodes of the discharge groups '89. andI813. This arrangement has the advantage over Fig. 8 of supplying astill more constant flow of energy to the alternating current circuit2!.

Heretofore in electric valve frequency converting systems it has beencommon to control the activities of the various arc discharge paths by acontrol circuit having two control components one corresponding to theinput frequency and the other corresponding to the output frequency.Such an arrangement operated to prevent certain arc discharge paths frombecoming conductive so that only those anodes having the proper phaseThe voltages for production of the alternating current ing of the outputtransformer. Therefore, in the arrangement shown in Figs. 8, 9 and 11 itis only necessary to provide three are discharge paths for eachpolyphase network group since each of the arc discharge paths isrendered conductive twice during each half cycle of alternating currentin the output circuit. Thus it will be seen that each discharge occursover a considerable period thereby providing a good utilization of thetransformer and yet the entire electric valve apparatus operates onlyintermittently thus providing good operation with reference to thenecessity for dissipating the heat with the result that the danger ofare back is materially reduced. The harmonic content of the electricvalve converting apparatus by a three phase converter utilizing myinvention is about equal to that of a six phase converter of the usualtype.

If it is desired further to improve the shape of the low frequency wave,then this can be accomplished by utilizing a greater number of phasesfor example six phases. The output voltage'will therefore be composed ofseveral partial voltages which have equal or different phase positions.It will be apparent from Fig. 3 that the addition of additional phasesare similarly dependent upon whether one considers either the upper orthe lower limits of the envelope Eu. Thus in a six phase rectifier onegroup of three phases will supply an envelope corresponding to U2. andthe other group will supply an envelope corresponding to Up.

The systems disclosed in Figs. 7, 8, 9, l0 and 11 are capable ofproducing a constant frequency alternating current output since it isrelatively easy to maintain the speed of these generators constant inview of the fact that the frequency of the output current fu is equal tofrom which it is apparent that any variation of the generator onlyproduces half as great a variation in the output frequency. If it isdesired to produce a variable frequency alternating current output itfurthermore will be apparent that the arrangements disclosed in thesefigures provide a simple arrangement for accomplishing this. Sucharrangements have numerous advantages over the arrangements in the priorart since at all output frequencies the power factor of the converter isrelatively high. Such arrangements furthermore have the advantage ofproducing symmetrical alternating current outputs in contrast to the,

asymmetrical wave shape produced by ordinary electric valve translatingmeans which have rectangular or trapezoidal alternating current waveshapes. While the frequency relation between the input frequency and theoutput .frequency is not an integral factor it is apparent that in themore simple systems the output waves are not symmetrical in shape.Symmetry in the conventional arrangements can only be approximated byutilizing a large number of'phases in a large number of arc dischargepaths so that each wave in the output circuit is produced by a pluralityof partial wave components supplied by the different electric dischargepaths. From this it will be apparent that the arrangements disclosed inaccordance with my invention have a relatively high power factor,relatively low harmonic components, and produce symmetrical alternatingcurrent wave outputs for both rigid frequency relation and variablefrequency relation between the input and output circuits."

' winding 43.

While my invention has been shown as applied to a frequency changingsystem operating between a polyphase alternating current source and asingle phase load circuit, it is equally applicable to transmission ofenergy between two polyphase alternating current circuits. An example ofsuch an arrangement is disclosed in Fig. 13 wherein an alternatingcurrent source 22 having a frequency fa and a second alternating currentsource 23 having a frequency fb supplies energy to an I electric valveconverting system connected to the two phase-alternating currentcircuits 24 and 25; One of the alternating current supply circuits maybe energized from a source of energy comprising a generator such asshown in Figs. '7 to 11. The alternating current source 22 is connectedto energize a transformer having a primary winding 26 and two secondarywindings 2'! and 28. The alternating current source'23 is connected toenergize a transformer having a primary winding 29 and a plurality ofsecondary windings 30 to 33. Two groups of electric discharge valves 34and 35 are provided to supply energy to the primary winding of thetransformers 36 and 31 which are connected, respectively, to the outputcircuits 24 and 25. The secondary winding 2'! of the one transformer andthe secondary windings 30 and 3| are connected together so that thewinding 21 is in series with the winding 30 and supplies energy throughthe valve group 34 and the primary winding of the transformer 26through'a suitable smoothing reactor 36. Winding 21 connected in serieswith the winding 3| supplies energy through the group of valves 35through a suitable smoothing reactor 31 to the primary. winding of thetransformer 21 The winding 28 in series with the' winding 32 suppliesenergy through the group of valves 34 and the smoothing reactor 36 tothe primary winding of the transformer 26 'and the winding 28 in serieswith the winding 33 supplies energy through the group of valves 35through the smoothing reactor 31 to the primary winding of thetransformer 21. The alternating currents appearing across the circuits24 and 25 are out of phase and ob- I viously if desired these may beconnected to a T- cennected transformer to supply threephase current. Itwill be apparent to those skilled in the art that any of thearrangements disclosed hereconstant and the frequency of the otheralternating current circuits such as 23 is variable the resultantfrequency appearing-in either of the output circuits 24 or 25 will alsobe variable.

Still another manner of obtaining a variable frequency output would beto vary the frequency of both the input circuits since the frequency ofthe alternating current load circuit corresponds to a frequency which ishalf of the difference between the frequency of the input circuit. Thusin order to produce a variable frequency output current it -is onlynecessary to vary the frequency rela- 'tion between the two alternatingcurrent supply circuits.

In Fig. 14 there is shown an arrangement for transmitting energy betweentwo three phase systems. Two alternating current supply lines 40 and 4|are'provided, one of which may be energized from a generator. Thealternating current circuit 40 .is connected to a transformer having aprimary winding 42 and asix phase secondary The alternating currentcircuit 4| is connected to a transformer having a primary winding 44 anda plurality of three phase secondary windings 45 to 50. Each of theouter extremities of the phase windings of the star-com nectedseconudary transformer winding 43 is connected to the neutral point of adifferent one of the secondary transformer windings 45 to 5.

Each of the outer extremities of the secondary windings 45 to 50 isconnected to two different electric valve groups. Six groups of electricvalves to 56 each having six are discharge paths are provided and eachgroup is connected through a suitable" smoothing reactor 5i to 52,respectively, to the outer extremities of a six phase star-connectedprimary winding 53 of a transformer the secondary winding 54 of which isconnected to the alternating current output circuit 65. For the purposesof simplicity only one of each of the outer extremities of the sixsecondary transformer windings 45 to 50 has been shown connected to thetwo valve groups St to 55 since it will be apparent to those skilled inthe art that the remaining transformer terminals are similarly connectedto the valve groups 52, 55; 53, 54.

While my invention has been shown and described in connection withcertain specific embodiments it will, of course, be understood that itis not to be limited thereto, since it is apparent that the principlesherein disclosed are susceptible of numerous other applications, andmodifications may be made in the circuit arrangements and in theinstrumentalities employed without departing from the spirit and scopeof my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

l. The method of producing a lower frequency alternating currentquantity from two alternating current quantities of higher frequencywhich comprises vectorially adding together said higher frequencyalternating current quantities to produce a resultant alternatingcurrent quantity having a variable frequency and a variable amplitude,and converting said resultant quantity to produce an alternating currentquantity corresponding to the envelope. of said resultant quantity.

2. The method of transmitting energy from a higher frequency alternatingcurrent circuit to a lower frequency alternating current circuit whichcomprises generating an alternating current having an amplitude and awave shape similar to the amplitude and wave shape of said source ofalternating current, the frequency of said generated alternatirficurrent being greater than that of said lower frequency alternatingcurrent circuit, adding together vectorially said generated alternatingcurrent with energy from said higher frequency alternating currentcircuit, and converting said resultant alternating current to produce analternating current having a frequency equal to that of said lowerfrequency alternating current circuit.

3. The method of producing a lower frequency alternating currentquantity from two alternating current quantities having higherfrequencies, similar wave shapes and similar amplitudes which comprisesadding vectorially said alternating current quantities of higherfrequency to produce a resultant alternating current quantity ofvariable frequency and variable amplitude, and converting said resultantvalue to produce an alternating current value having a wave shapecorresponding to the envelope of said resultant quantity.

4. A method which comprises adding vectorially two alternatingcurrentquantities of different frequency but of similar amplitude and similarwave shape to produce a resultant alternating current quantity ofvariable frequency and variable amplitude, varying the frequencyrelation between said higher frequency alternating current quantities,and rectifying and converting said resultant alternating currentquantity to produce a variable frequency alternating current quantityhaving a wave shape corresponding to the envelope of said resultantalternating current quantity.

5. An electric frequency changing system comprising a source ofalternating current having a certain frequency, amplitude and waveshape, means energized from said source for producing an alternatingcurrent of different frequency and similar amplitude and wave shape, andmeans energized from said latter means and from said source forcombining said alternating currents to produce a resultant alternatingcurrent of variable frequency and variable amplitude.

6. An electric valve frequency converting system comprising two sourcesof alternating current of different frequencies and of similar waveshape and similar amplitude, means for adding together said alternatingcurrents, electric valve converting means energized from said lattermeans for producing an alternating current having a frequency equal tohalf of the difference between the frequencies of said first mentionedalternating currents.

7. In combination, an electric valve frequency changing apparatusincluding a plurality of arc discharge paths and two groups of seriallyconnected polyphase inductive networks, an alternating current outputcircuit for said apparatus, said output circuit having a predeterminedfrequency, and means for supplying energy to said groups of polyphaseconnected networks, said energy comprising two alternating currentshaving similar wave shapes and like amplitudes and differing infrequency by an amount equal to twice the frequency of said alternatingcurrent output circuit.

8. The combination comprising an electric valve corr'erting apparatusincluding a plurality of arc discharge valves and two groups of seriallyconnected polyphase inductive networks, means for energizing one of'saidinductive networks from a source of polyphase alternating current havinga certain frequency amplitude and wave shape, means for energizing theother of said inductive networks with alternating current of similarwave shape and amplitude and means for varying the frequency relation ofthe alternating currents supplied to said networks.

9. The combination comprising an electric valve frequency convertingsystem including a plurality of arc discharge devices, two groups ofpolyphase inductive networks, said networks being connected in seriesrelation, an output circult having a predetermined frequency, and meansfor energizing each of said inductive networks with alternating currenthaving similar wave shapes and amplitudes, said alternating currentsdiffering in frequency by an amount equal to twice the frequency of saidalternating current output circuit.

10. An electric frequency changing system lit) comprising a source ofalternating current having a predetermined frequency, amplitude and waveshape, means energized from said source for producing a secondalternating current of difierent frequency and similar amplitude andwave shape, means energized from said latter means and from said sourcefor vectorially combining said alternating currents of difierentfrequencies to produce a resultant alternating current of variablefrequency and variable amplitude, and electric valve frequency coveringmeans energized by said resultant alternating currents for producing analternating current having a frequency corresponding to the frequency ofthe envelope of said resultant alternating current.

WAIDEMAR PETERSEN.

Certificate of Correction Patent No. 2,178,432. October 31, 1939.WALDEMAR PETERSEN It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows: Page 1, first column, line 20, for the word system readsystems; and second column, line 46, in the formula, for E sin W read E=A sin w i; page 2, second column, line 31, for 3A read 2A; page 3,first column, l1ne 18, for E,,, E,, read E, and E,,; and second column,line 44, for (3 +3 read ($w,,+8,,)+; page 5, first column, line 5, forresistor read resistance; page 6, second column, line 47, for cennectedread connected; page 7, first column, line 5, for seconudary readsecondary; and second column, line 59, claim 8, after amplitude insert acomma; page 8, second column, line 2, claim 10, for covering readconverting; and that the said Letters Patent should be read with thesecorrections therein that the same may conform to the Word of the case inthe Patent Office.

Signed and sealed this 12th day of December, A. D. 1939.

HENRY VAN ARSDALE,

Acting Commissioner of Patents,

Certificate of Correction Patent N 0. 2,178,432. October 31, 1939.WALDEMAR PETERSEN It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows: Page 1, first column, line 20, for the word system readsystems; and second column, line 46, in the formula, for E sin W read E=A sin w h; page 2, second column, line 31, for 3A read 2.4; page 3,first column, lme 18, for E',,, E,, read E, and E";,; and second column,line 44, for (3 '+3 read (8w +3 page 5, first column, line 5, forresistor read resistance; page 6, second column, line 47, for connectedread connected; page 7, first column, line 5, for seconudary readsecondary; and second column, line 59, claim 8, after amplltndeli inserta comma; page 8, second column, line 2, claim 10, for covering readconverting; and that the said Letters Patent should/be read with thesecorrections therein that the same may conform to the of the case in thePatent Office. Signed and sealed this 12th day 0f December, A. D. 1939.

HENRY VAN ARSDALE,

Acting Commissioner of Patents,

